Trout Population Effects Of Water Flow & Levels

I have a lot of theories about fly fishing and fish that I have developed over the years that come from hundreds of days on the water and hours thinking about the various factors that influence fishing and fish. Some of these have no scientific backing but are just based on observations, some are just based on hypothetical scenarios, others I have actually studied scientific research to satisfy my curiosity. At any rate, they may be pure BS or have some true merit. I do enjoy theorizing and discussing this type of thing with people who are interested which is why I wrote this post.

Basically, I wanted to mention some of my theories and what I know about fish populations and how water levels and flows affect them. A friend was asking me what I thought about how recent high water years and the abnormally high flows in the west effect the trout populations. He figured it was a good thing which in general it is. However, considering that a very high percentage of our western fisheries are tailwaters or influenced by dams or irrigation that results in un-natural manipulated flows at odd times, the effects vary quite a bit. I don’t want to get too deep, but just present a few basic thoughts.

High Flows = Higher Trout Population?

This is a fairly common perception and for the most part it holds true. If there is more water in the river throughout the year, then the population would very likely do better. This is because of increased areas for small fish to winter over and hide from predation especially during young life stages. I have both witnessed this first hand and read scientific accounts of where this is a key factor in trout populations. In addition, with higher flows, the water temperature can remain cooler and more oxygenated in the later summer months and insects have better habitat for increased food supply.

High Flows At Odd Times = Unpredictable Effects

If the high flows occur during times when high flows would normally not occur, odd things can happen. If for example on a brown trout fishery (most of our local fisheries are), abnormally high flows occur in the winter, the fish are fighting higher water at a time when they are weaker after the spawn. This can result in slower winter growth and less healthy fish that take on a “snaky” look with bigger heads and skinny bodies. This is especially true when the water is released from a dam at very cold temperatures which keeps the trout metabolism low so they are not very active so they don’t feed much and have to fight the higher flow. There are times though when the high flows can benefit. If the water is reasonably warmer and the flows are consitent, then there can be increased food available and more habitat in the form of sidechannels and fish can actually do better. It all depends on the combination of factors.

High Flows = Healthier Fish?

This is where things get interesting. Many people figure that if there is more water, then conditions are better throughout the year so fish become healthier. There is a lot of truth to this for similar reasons mentioned above (cooler summer temps, better feeding conditions). If you have more optimum conditions year round, then you have better growth conditions. The issue arises though as you have optimum conditions and fish populations continue to rise without any significant mortality, the trout can overpopulate and thereby out-compete each other for the food source. This results in a higher population of less healthy, smaller fish and very few bigger fish.

Best Scenario = Balance

The best case scenario is just the right flow at the right time, in essence a natural un-dammed river not in a drought scenario. In this scenario the water is high in spring-time during runoff and gradually drops throughout the summer until it reaches a lower late summer flow. In a good water year (at or a little above average snowpack), the water gets low and late snowmelt keeps it from getting too low and the winter flow gets low but not so low that juvenile recruitment is hampered, but enough juvenile predation occurs to keep the population in balance. This results in the right balance and healthy fishery for the most part. Getting the balance just right though is a very tenuous issue as there are so many factors involved.

Of course these are just broad generalizations, but there are conclusions you can draw from them when applied to any trout fishery. I try to keep the history of flows in mind as I choose where to fish at what time throughout the year. Sometimes everything comes together on a spot where normally it isn’t that good and you have spectacular fishing for big fish with nobody around. That is the glory that I am always looking for!